Graphene's Thermal Conductivity

The structure that fills this SEM image was devised to measure thermal conductivity (hotter regions are colored red; cooler regions, blue) in a single sheet of graphene (inset) supported on a silicon dioxide beam.

The structure that fills this SEM image was devised to measure thermal conductivity (hotter regions are colored red; cooler regions, blue) in a single sheet of graphene (inset) supported on a silicon dioxide beam.

The heat conductivity of the advanced material graphene when it is in contact with other materials—the way it is likely to be used in future nanoelectronics applications—has been a mystery until now. A new study in Science finds that graphene is an exceptional heat conductor when the one-atom-thin sheet of carbon is draped across a solid (2010,328, 213).

The investigation may hasten development of graphene-based electronic devices by presenting an experimental method for measuring—and a theoretical framework for understanding—heat generation and dissipation in electronic circuitry that includes components made from graphene.

A collection of useful properties, including outstanding electrical conductivity and mechanical strength, have made graphene a much-studied material for nanoscale electronics applications (C&EN, March 2, 2009, page 14). As devices continue to shrink and circuit density increases, high thermal conductivity, which is essential for dissipating heat efficiently to keep electronics cool, plays an increasingly larger role in device reliability. Yet because of experimental difficulties, few studies of graphene’s thermal conductivity have been done.

Various forms of carbon, including diamond, graphite, and carbon nanotubes, are excellent thermal conductors as a result of their structure, bonding, and low mass. According to an earlier study, when graphene is suspended freely (like the middle part of a bedsheet hanging between a pair of clotheslines), this carbon allotrope also exhibits extreme thermal conductivity (κ). That study places suspended graphene’s κ-value as high as 5,000 W per meter per Kelvin—2.5 times greater than that of diamond, which holds the record among naturally occurring materials. But in useful applications, graphene is likely to lie in contact with other materials.

To measure its κ-value under that condition, University of Texas, Austin, scientists Jae Hun Seol, Li Shi, Rodney S. Ruoff, and coworkers devised a microscale electronic thermometer and used it to measure the thermal properties of two types of samples. First the team analyzed composites consisting of a single layer of graphene deposited on silicon dioxide, a standard support material. Then they etched away the graphene and reanalyzed the remaining samples’ thermal properties.

From the difference, the team determined that the thermal conductivity (near room temperature) of supported, single-layer graphene is about 600 W per meter per Kelvin. That κ-value is almost an order of magnitude lower than the value for suspended graphene. Yet it’s also, respectively, about two times and 50 times as great as the values for copper and silicon, two materials widely used in today’s microelectronics.

The researchers also applied computational methods to better understand the differences between the thermal properties of suspended and supported graphene. They found that the lack of perfect conformity between graphene and the underlying solid, due to the solid’s nanoscale roughness, causes graphene’s phonons—collective lattice vibrations—to “leak” into the solid, thereby reducing the κ-value of supported graphene relative to that of suspended graphene.

“What makes this study remarkable,” says Ravi Prasher, a microprocessor-cooling specialist at Intel, “is that it combines thermal, structural, and mechanical phenomena into one theoretical framework.” Prasher, who wrote a commentary on the work for the same issue of Science, adds that the study is “a crucial first step” toward explaining the thermal conductivity of supported graphene. He notes, however, that a host of issues regarding graphene’s thermal properties remain to be explored.